A typical dynamic headphone unit includes a permanent magnet, a pole piece, and a yoke. The pole piece is disposed adjacent one of the magnetic poles of the permanent magnet, whereas the yoke is disposed adjacent the other magnetic pole of the permanent magnet. The permanent magnet, the pole piece, and the yoke define a magnetic circuit having a magnetic gap. The magnetic gap accommodates a voice coil vibratably supported by a diaphragm. If current flows through the voice coil in response to audio signals, the interaction between the magnetic field caused by the current and the magnetic field in the magnetic gap vibrates the voice coil. The vibrations of the voice coil correspond to the audio signals. The diaphragm vibrates with the voice coil. That is, the diaphragm vibrates in response to the audio signals to output sound. The dynamic headphone unit further includes a line (hereinafter referred to as “lead line”) for transmitting the audio signals to the voice coil.
The diaphragm of the dynamic headphone unit has a large displacement. The voice coil is fixed to the diaphragm; hence, the displacement of the diaphragm leads to a large displacement of the lead line being a part of the voice coil. The lead line is thin and thus is readily broken by a large displacement. In other words, the lead line is readily subject to disconnection. If the lead line is not firmly fixed, the lead line vibrates like a skipping rope in response to the vibrations of the diaphragm. Such vibrations of the lead line cause noise.
FIG. 4 is a cross-sectional view of a conventional dynamic headphone unit along the direction of displacement of the diaphragm. In FIG. 4, a magnetic circuit in the dynamic headphone unit is not depicted. As illustrated in FIG. 4, the dynamic headphone unit includes a diaphragm 11, which includes a central dome 12 and a peripheral dome 13 around the central dome 12. A voice coil 14 is fixed in the vicinity of the boundary between the central dome 12 and the peripheral dome 13.
A lead line 15 is connected to a portion of the voice coil 14. The lead line 15 is also connected to a signal input circuit (not shown). The lead line 15 is connected to the signal input circuit at a node A, and is connected to the voice coil 14 at a node B. The vibratory lead line 15 can readily be disconnected at the nodes A and B. The lead line 15 sometimes vibrates like a skipping rope around the nodes A and B. Such vibrations of the lead line 15 cause noise.
In order to prevent the disconnection of and the noise from the lead line, the vibrations of the lead line should be reduced. In a known dynamic headphone unit, lead lines connected to a wire end of the voice coil are twisted and thus strengthened (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-153383). In another known dynamic headphone unit, the lead line is fixed to the inner surface (rear surface) of the peripheral dome such that the lead line does not vibrate.
The lead line is fixed to the rear surface of the peripheral dome, for example, by bonding the lead line to the peripheral dome with an adhesive. The bonded state will now be described. FIG. 5 illustrates an exemplary conventional dynamic headphone unit in which a lead line is firmly bonded to the rear surface of a peripheral dome. With reference to FIG. 5, an adhesive 17 is applied to the peripheral dome 13 to bond the lead line 15 to the peripheral dome 13. A corrugation 18 is formed on the peripheral dome 13.
In a conventional technique to fix the lead line as illustrated in FIG. 5, a part of the lead line 15 extends over or intersects some corrugations 18 at the outer periphery of the peripheral dome 13. Such a conventional technique to fix the lead line 15 can reduce the loads on the lead line 15 caused by the displacement of the peripheral dome 13. The lead line 15 is fixed to the peripheral dome 13 through the curing of the adhesive 17. The adhesive 17 thus stiffens a part of the peripheral dome 13 that is bonded to the lead line 15 (coated with the adhesive 17).